1. FIELD OF THE INVENTION
This invention relates to a novel process for preparing a phosphonitrile oligomer having a substituted hydroxyl group. More particularly, the present invention is concerned with a novel process for preparing a mixture of a phosphonitrile oligomer represented by the following general formula: ##STR1## (wherein n is an integer of 3 or more, and
R is
(i) a straight-chain or branched chain alkyl group containing or not containing a halogen atom or an alkoxy group, PA1 (ii) a straight-chain or branched chain alkenyl group, PA1 (iii) a straight-chain or branched chain alkinyl group, PA1 (iv) an aralkyl group containing or not containing a halogen atom, an alkyl group, or an alkoxy group, PA1 (v) a cycloalkyl group) PA1 .circle.1 a straight-chain or branched chain alkyl group containing or not containing a halogen atom or an alkoxy group, PA1 .circle.2 a straight-chain or branched chain alkenyl group, PA1 .circle.3 a straight-chain or branched chain alkinyl group, PA1 .circle.4 an aralkyl group containing or not containing a halogen atom, an alkyl group, or an alkoxy group, or PA1 .circle.5 a cycloalkyl group), PA1 triethylamine, PA1 tripropylamine, PA1 tributylamine, PA1 trioctylamine, PA1 N-methyl-N-ethylpropylamine, and PA1 N,N-diethylpropylamine; PA1 N,N-dimethylaniline, PA1 N,N-diethylaniline, PA1 N,N-diethyltoluidines; PA1 N,N-dimethylbenzylamine; and PA1 triethylenediamine, PA1 quinuclidine, PA1 N-methylpyrrolidine, PA1 N-methylpiperidine, PA1 N,N'-dimethylpiperidine, PA1 pyridine, PA1 alpha-picoline, PA1 beta-picoline, PA1 gamma-picoline, PA1 5-ethyl-2-picoline, and PA1 trimethylpyridine.
with a poly(phosphonitrile oligomer) obtained by condensing said phosphonitrile oligomer.
The poly(phosphonitrile oligomer) is a compound represented by, e.g., the following general formula: ##STR2## and is finding industrial applications including uses as flame retardants, heat-resistant agents, lubricating agents, and electro-insulating agents, etc.
2. DESCRIPTION OF THE PRIOR ART
Alkoxyphosphonitrile oligomers and poly(alkoxyphosphonitrile oligomers) produced by condensing said alkoxyphosphonitrile oligomers are finding wide applications including use in various industrial materials by virtue of their excellent heat-resistance, cold-resistance, lubricity, electro-insulation and chemical stability.
The said alkoxyphosphonitrile oligomer is obtained by alkoxylation of phosphonitrile chloride oligomer having a cyclic or linear structure represented by the formula: ##STR3## (wherein n is defined above), particularly cyclic phosphonitrile chloride oligomers represented by the formula: ##STR4## (wherein n is 3), and cyclic phosphonitrile chloride oligomers represented by the formula: ##STR5## (wherein n is 4) and mixture composed mainly of said two kinds of phosphonitrile chloride oligomer.
Further, a mixture of an alkoxyphosphonitrile oligomer with a poly(alkoxyphosphonitrile oligomer) has drawn particular attention as a flame retardants for rayon and other various organic high-molecular substances.
Various processes have been proposed for producing such a mixture. Examples of the processes include (1) one as described in Japanese Patent Laid-Open No. 109320/1974 which uses a tertiary amine as an acid acceptor and (2) one as described in Japanese Patent Laid-Open No. 216895/1984 (corresponding to U.S. Pat. No. 4,571,310) which is an improved process of the above process (1).
(1) Process described in Japanese Patent Laid-Open No. 109320/1974:
This process comprises reacting a phosphonitrile chloride oligomer with an alcohol in the presence of an acid acceptor comprised of a tertiary amine, such as pyridine or triethylamine, to produce a compound in which a chlorine atom still remains in the nucleus of the phosphonitrile chloride oligomer, i.e., a partially alkoxylated phosphonitrile oligomer, and heating this compound in the presence of a tertiary amine to form a linkage represented by the abovementioned general formula (II).
In this process, the amount of the remaining active chlorine which is still contained in the alkoxyphosphonitrile oligomer at the stage of the completion of the first step is small (e.g., about 1% to about 15%). Therefore, in order to complete the condensation reaction in the presence of the tertiary amine in a short time (e.g., 5.5 hr), it is necessary to conduct the condensation reaction at a high temperature (e.g., about 100.degree. C.) (see Examples 1, 4, 5, and 6). This makes it impossible to suppress the excessive condensation. On the other hand, the practice of the condensation reaction at a low temperature (e.g., about 30.degree. C. to about 35.degree. C.) is disadvantageous from the standpoint of industry, because it takes a prolonged period of time, i.e., as long as 18 hrs (see Example 2). In this process, when the temperature of the condensation reaction exceeds 60.degree. C., a substance in which an OH group is bonded to a phosphorus atom in the phosphonitrile is produced as a by-product, which deteriorates the quality of the final product as will be mentioned later.
(2) Process described in Japanese Patent Laid-Open No. 216895/1984:
This process aims at producing a phosphonitrile compound having an alkoxy group using phosphonitrile chloride oligomer and an alcohol as starting materials, and comprises:
(A) a first step of making the phosphonitrile chloride oligomer react with "an" alcohol in an amount of 0.2 to 0.8 equivalent per equivalent amount of the active chlorine contained in the phosphonitrile chloride oligomer in the presence of an organic tertiary amine in an equivalent amount or more relative to the amount of the alcohol at a temperature of 20.degree. C. or below to substitute 34 to 75% of the active chlorine by an alkoxy group,
(B) a second step of elevating the temperature of the reaction mixture obtained in the first step to cause a condensation reaction through the elimination of an alkyl chloride, and
(C) a third step of adding an alcohol to the reaction system obtained above in an at least equivalent amount relative to the amount of the active chlorine which remains at the stage of completion of the second step to the reaction system obtained in the second step to cause a reaction of the alcohol with the active chlorine, thereby the remaining active chlorine is substantially completely substituted by an alkoxy group.
According to this method, the amount of the remaining active chlorine in the alkoxyphosphonitrile at the stage of completion of the first step is large, which not only makes it possible to smoothly progress the condensation reaction in the second step even at a temperature of 60.degree. C. or below but also makes it possible to progress the condensation reaction while determining the degree of condensation and to cease the condensation reaction at will based on the result of the determination. However, in this method, products which are considerably easily soluble in water are frequently produced, although this method intends to produce a phosphonitrile compound having an extremely low solubility in water. This is because the reaction product contains a large amount of an oligomer comprised of a phosphonitrile unit having an OH group represented by the following formula: ##STR6## The oligomer having an OH group such as one represented by the above formula (VI) has a high solubility in water, which renders the product containing it considerably soluble in water. When a mixture containing a phosphonitrile compound which is considerably easily soluble in water is added to, e.g., a viscose solution in preparing a nonflammable rayon, part of the phosphonitrile oligomer or poly(phosphonitrile oligomer) having an OH group is dissolved in a spinning bath, which makes it impossible to impart an intended flame retardance to the rayon. Further, the rayon containing a phosphonitrile compound which is considerably easily soluble in water is unfaborable for practical use, because the flame retardance is remarkably lowered when it is repeatedly washed.